The present invention is generally directed to processes for the preparation of toners, and more specifically to economical one step direct processes for modifying toner resin characteristics, and the preparation of toner compositions thereafter. More specifically, The present invention relates to melt mixing processes, batch or continuous, and preferably continuous processes such as, for example, extrusion for the preparation of toner compositions, and wherein the toner resin is comprised of certain crosslinked fraction generated during toner preparation, reference copending patent application U.S. Ser. No. 814,641 (D/91117) and U.S. Pat. No. 5,227,460 (D/91117Q), the disclosures of which are totally incorporated herein by reference, and which applications illustrate, for example, melt mixing processes for preparing toner including the first step of a reactive melt mixing process to crosslink a base resin and the second step of a melt mix process to prepare a toner from the crosslinked resin by incorporating toner additives. With the processes of these copending patent applications, two melt mixing steps are selected to prepare a toner from a base resin. The present invention combines the reactive melt mixing and the toner additive mixing in a one step process. Thus, with the processes of the present application, only one melt mixing step or one operation through a melt mixing device is needed for preparing toner directly from a base resin, and which process offers economical advantages. The toner prepared in accordance with the processes of the present invention can be selected for heat fixable imaging and printing, such as xerographic methods, and wherein there results excellent fusing and vinyl offset performance.
Toner utilized in development in the electrographic process is generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, several polymers are known including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, styrene-butadiene resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black is utilized often, and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and the like are known.
Toner can be fixed to a support medium such as a sheet of paper or transparency by different fixing methods. A fixing system which is very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes is hot roll fixing. In this method, the support medium carrying a toner image is transported between a heated fuser roll and a pressure roll with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium forming a fixed image.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as the cold offset temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is known as the hot offset temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images, resulting for example in blurred images. This undesirable phenomenon is called offsetting. Between the COT and HOT of the toner is the minimum fix temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is called the fusing latitude.
The hot roll fixing system described above and a number of toners presently used therein exhibit several problems. First, the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Second, offsetting can be a problem; third, toner containing vinyl type binder resins such as styrene-acrylic resins may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl binder covers, and the fixed image adheres to the PVC surface.
There is a need for a toner prepared by simple economical one step processes which has a low fix temperature and a high offset temperature (or wide fusing latitude), and superior vinyl offset property. Toners which operate at lower temperatures would reduce the power needed for operation and increase the life of the fuser roll and the high temperature fuser roll bearings. Additionally, such low melt toners, that is, for example, toners having an MFT lower than 200.degree. C., and preferably lower than 160.degree. C., would reduce the volatilization of release oil such as silicone oil which may occur during high temperature heating operation and which can cause problems when the volatilized oil condenses in other areas of the machine. In particular, toners with a wide fusing latitude and with acceptable toner particle elasticity are needed. Toners with wide fusing latitude can provide flexibility in the amount of oil needed as release agent and can minimize copy quality deterioration related to toner offsetting to the fuser roll.
To lower the minimum fix temperature of the binder resin, in some instances the molecular weight of the resin may be lowered. Low molecular weight resins, such as amorphous polyester resins and epoxy resins, have been used for low fixing temperature toners. For example, the use of polyester resins as a toner binder is disclosed in U.S. Pat. No. 3,590,000 to Palermiti et al. and U.S. Pat. No. 3,681,106 to Burns et al. The minimum fixing temperature of polyester binder resins can be lower than that of other materials, such as styrene-acrylic and styrene-methacrylic resins. However, this may lead to a lowering of the hot offset temperature, and as a result decreased offset resistance. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage.
To prevent fuser roll offsetting and to increase fusing latitude of toners, various modifications of binder resin structure have been made, for example, by branching or crosslinking. In U.S. Pat. No. 3,681,106 to Burns et al., for example, a polyester resin was improved with respect to offset resistance by nonlinearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
Another method of improving offset resistance is to utilize crosslinked resin in the binder resin. For example, U.S. Pat. No. 3,941,898 to Sadamatsu et al. discloses a toner in which a crosslinked vinyl type polymer is used as the binder resin. Similar disclosures for vinyl type resins are made in U.S. Pat. Re. No. 31,072 (a reissue of U.S. Pat. No. 3,938,992) to Jadwin et al., U.S. Pat. No. 4,556,624 to Gruber et al., U.S. Pat. No. 4,604,338 to Gruber et al. and U.S. Pat. No. 4,824,750 to Mahalek et al.
While significant improvements can be obtained in offset resistance, a major drawback may ensue in that with crosslinked resins prepared by conventional polymerization, that is crosslinking during polymerization using a crosslinking agent, there exist three types of polymer configurations: a linear and soluble portion referred to as the linear portion, a portion comprising highly crosslinked gel particles which is not soluble in substantially solvents, like tetrahydrofuran, toluene and the like, and is called gel, and a crosslinked portion which is low in crosslinking density and therefore is soluble in some solvents, such as, tetrahydrofuran, toluene and the like, and is referred to as sol. The presence of highly crosslinked gel in the binder resin increases the hot offset temperature, but at the same time the low crosslink density portion or sol increases the minimum fix temperature. An increase in the amount of crosslinking in these types of resins results in an increase not only of the gel content, but also of the amount of sol or soluble crosslinked polymer with low degree of crosslinking in the mixture. This results in an elevation of the minimum fix temperature, and as a consequence, in a reduction or reduced increase of the fusing latitude. Also, a drawback of embodiments of crosslinked polymers prepared by conventional polymerization is that as the degree of crosslinking increases, the gel particles or very highly crosslinked insoluble polymer with high molecular weight grow larger. The large gel particles can be more difficult to disperse pigment in causing the formation of unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, compatibility with other binder resins may be relatively poor and toners containing vinyl polymers often show vinyl offset.
Crosslinked polyester binder resins prepared by conventional polycondensation reactions have been provided for improving offset resistance such as, for example, in U.S. Pat. No. 3,681,106 to Burns et al. As with crosslinked vinyl resins, increased crosslinking as obtained in such conventional polycondensation reactions may cause the minimum fix temperature to increase. When crosslinking is effected out during polycondensation using tri- or polyfunctional monomers as crosslinking agents with the polycondensation monomers, the net effect is that apart from obtaining highly crosslinked high molecular weight gel particles, which are not soluble in substantially any solvent, the molecular weight distribution of the soluble part widens due to the formation of sol or crosslinked polymer with a very low degree of crosslinking, which is soluble in some solvents. These intermediate molecular weight species may result in an increase in the melt viscosity of the resin at low and high temperature, which can cause the minimum fix temperature to increase. Furthermore, gel particles formed in the polycondensation reaction which is carried out using conventional polycondensation in a reactor with low shear mixing can grow rapidly with increase in degree of crosslinking. As in the case of crosslinked vinyl polymers using conventional polymerization reactions, these large gel particles may be more difficult to disperse pigment in, resulting in unpigmented toner particles after pulverization, and thus hindering developability.
Crosslinked polyester binder resins prepared by a reactive melt mixing process have been disclosed in U.S. Pat. N. 5,227,460 (D/91117Q), the disclosure of which is totally incorporated herein by reference. In this process, the crosslinking reaction is accomplished with a chemical initiator when the polymer is in the molten state. The crosslinked resin comprises crosslinked portions and linear portions. The crosslinked portions comprise very high molecular weight densely crosslinked gel particles having average diameter less than about 0.1 micron and are insoluble in substantially any solvent. The linear portion comprises low molecular weight resin soluble in various common solvents. Substantially no portion of the resin comprises sol or polymer with low degree of crosslinking. The crosslinked portions or microgel particles are prepared in such a manner that there is substantially no distance between the polymer chains. This crosslink structure is different from conventional crosslinking in which the crosslink distance between chains is quite large with several monomer units. The highly crosslinked microgel particles distributed throughout the linear portion impart elasticity to the resin, which improves the resin offset properties, while not substantially effecting the resin minimum fix temperature. This melt mixing process in U.S. Pat. No. 5,227,460 (D/91117Q) is a reactive melt mixing process whereby a base resin is converted into a resin with crosslinked fraction.
Many processes are known for effecting polymerization reactions, including reactive melt mixing processes, for both initial polymerization reactions employing monomers or prepolymers, and for polymer modification reactions, such as grafting, coupling, crosslinking and degradation reactions. The process is known as reactive extrusion process when the melt mixing device is an extruder. The reactive extrusion process is particularly advantageous for polymer modifications in many respects. The modification generally takes place when the polymer is in molten state, thus eliminating the use of a large amount of solvent whose handling is both difficult and costly. The extrusion process is inherently easier to control as compared to a large polymerization reactor vessel.
Melt mixing processes for preparing toner based on crosslinked polyester binder resin, such as those disclosed in U.S. Pat. No. 5,227,460 (D/91117Q), are illustrated in pending application U.S. Ser. No. 814,641 (D/91117), the disclosure of which is totally incorporated herein by reference. In these processes, polymers are crosslinked using chemical initiator as crosslinking agent in the molten state at high temperature in an extruder. The partially crosslinked resin prepared by the reactive extrusion process is subsequently melt blended again with a colorant, charge enhancing additives and the like to result in a toner mixture before pulverizing operation to obtain toner particles. Although the first reactive extrusion operation can prepare a toner resin comprising very high molecular weight densely crosslinked microgel particles which improve the resin offset properties, the need to subject the resin to a second extrusion operation wherein intensive mixing is employed to disperse toner additives will likely cause some of the gel particles to break down and thus narrow the fusing latitude. In addition, with the processes of these pending patent applications two melt mixing operations are required to first modify a base resin and then incorporate toner additives. These processes thus have the economical disadvantages typical of a two step process such as high operating cost and low production rate.
U.S. Pat. No. 5,057,392 to McCabe et al. discloses a low fusing temperature toner powder which employs a polyblend of a crystalline polyester and an amorphous polyester that has been crosslinked with an epoxy novolac resin in the presence of a crosslinking catalyst. The mixture, which includes the polyesters, the epoxy novolac resin, catalyst and colorant, is melt blended on heated compounding rolls or by passage through an extruder. During melt blending, the amorphous polymer is crosslinked with the epoxy novolac resin. Crosslinking substantially increases the offset latitude of the mixture. After melt blending, the mixture is annealed by being maintained at a temperature above the glass transition temperature of the amorphous polyester, but below the melting temperature of crystalline polyester, preferably in the range of 50.degree. to 80.degree. C. The annealing is continued for a time sufficient for the crystalline polyester to recrystallize as dispersed small particles within a matrix phase comprised of a crosslinked polymeric reaction product of the amorphous polyester and the epoxy novolac resin. Typical annealed times are in the range of about 0.2 to about 2 hours. If annealing is not carried out, the polyblend does not have the desired grindability and the toner powder derived therefrom does not have desired fusing temperature and keep characteristic. Though this patent claims a low fixing temperature toner, the toner composition is a specifically defined polymer blend including both amorphous and crystalline polyesters. Also, the melt blending and reaction process is not sufficient to provide a toner with desired properties. An additional annealing step, following melt blending, to recrystallize the crystalline polyester is necessary in order to provide the toner with optimum morphology. Another potential problem not addressed in the patent is the possibility of interference from some active toner additives during crosslinking. For instance, it is known that some carbon black pigments will inhibit certain types of polymer reactions.
Therefore, there remains a need for a toner composition and fabrication processes during which the toner properties can be optimized by a reaction, such as crosslinking in a simple straightforward manner.